The invention relates to hermetically sealed packages for solid state components, in particular for high frequency applications, and to a process for manufacturing such packages.
There are several different types of packages being used in the microwave industry for discrete small signal components and devices. These packages range from 6 GHz to 20 GHz in limiting frequency and vary from very high reliability ceramic packages to low cost plastic packages. However, to date there has not been a package available that combines all of the desirable features for such a package including hermetic sealing, small size, surface mount capability and microstrip compatibility, operating frequency up to 20 GHz and the ruggedness and reliability to meet the JAN-S requirements for environmental testing.
The problem has been that the requirements for these features tend to be mutually contradictory. To be hermetic and meet the JAN-S requirements, the package usually has to be fairly large in order to have good lead strength and to have room for a hermetic seal. Conversely, to operate up to 20 GHz, the package must be made very small to prevent cavity oscillation. Also, the parasitic capacitance from lead to lead must be very low, on the order of 0.05 pF, but the dielectric constants of hermetic ceramic package materials are fairly high, so line coupling must be kept to a minimum. To operate well in the 20 GHz range, the leads must be close to 50 ohm transmission lines, but this imposes physical and electrical constraints on the sealing material and makes hermeticity more difficult to achieve.
There are several packages that have some, but not all, of the features listed above. One prior solution is a package that has a ceramic base metalized on one side to which the leads are brazed. A beam lead diode is attached directly to the leads and a dot of epoxy is applied to cover the diode. This package has small inductance and capacitance and also small size but does not meet the JAN-S requirements and is not hermetic because the epoxy coating does not make a hermetic seal.
A second approach is a package using a co-fired ceramic base which has the cavity for holding the component. This construction provides good hermetic sealing and good lead strength, so it meets the environmental requirements for JAN-S testing. However, this construction requires the leads to be brazed to the backside of the base and a metalized seal ring for solder sealing a metal lid. Thus there are three metalized surfaces in the package, and the geometry of these surfaces creates relatively high parasitic inductance and capacitance because of excessive line coupling. Due to its high parasitic inductance and capacitance and large size, this type of package cannot be in applications above 12 GHz. It also has a relatively high piece part and manufacturing cost.
A third approach has a frequency range to above 18 GHz and meets the JAN-S level testing but is very expensive to manufacture, has a relatively high parasitic capacitance and inductance, and is difficult to test and mount in circuits. This package is a ceramic cylinder with metalization at both ends so that metal lugs can be soldered in. The device is mounted on one of the end lugs, with bond wires extending from the device to the second end lug. The second end lug is then soldered on to make the seal.
Yet another approach is a package with metalization on both sides of a ceramic base with the top metalization for mounting the device and the bottom metalization for brazing the leads. A lid with a cavity is attached to the top surface with a solder glass seal. This package is thus hermetic and meets the JAN-S environmental requirements but again suffers high parasitic inductance and capacitance and therefore cannot be used in applications above 12 GHz.